1. Field of the Invention
The invention relates to an alarm device for monitoring a space which is bounded at least on one side by a glass pane, a wall or any other boundary surface which will be referred to as a "barrier" hereafter. There exist many devices of this type, most of which trigger an alarm when an electrical circuit is interrupted, due to the breakage of the barrier.
2. Description of the Prior Art
This applies for instance to many protection systems for glass panes which comprise wires or a continuous conductive layer, and also to systems which use a so-called "spider" which is generally located in a corner of the pane.
These systems, also in general use, have a number of drawbacks such as:
the alarm is only triggered when the pane is broken because only this can modify the electrical characteristics of the monitoring circuit; in order to obtain this spider, one must use a thermally tempered glass, which entails optical disadvantages (insufficiently plane surface) and also an immediate and total crumbling of the tempered glass pane, PA1 there is no alarm when the frame rather than the pane is attacked, so that an aperture could be created by removing the glass pane from its frame, PA1 the wires can be seen and may hence in certain cases spoil the appearance and be aesthetically displeasing. PA1 it can trigger an alarm for a predetermined level of attack even if the pane is not broken at the first impact; this can become important when firearms are fired on a window pane because in this case an alarm will be triggered upon the very first impact, whether it destroys the pane or not; PA1 the alarm is also triggered when significant forces are exerted on the frame or when the circuit leading to the alarm center is broken; PA1 the device comprises no part which is either visible or impairs visibility, and it can be adapted both to single panes and to insulating panes (with two or more plies) because the pick-up can be embodied in the shape of a comparatively flat ribbon which can be easily fastened along the periphery of a protective surface such as a glass pane. The pick-up which is affixed to the pane is constructed so as to react efficiently and electrically either when any point of the pane is subjected to a sudden mechanical stress, or when the electrical field in the vicinity of the pane is modified. Due to its very construction, the device has no inertia but still it is passive and stable (no timedrift). Its linear shape and its small thickness enable it to be easily adapted to any type of pane without the necessity to essentially change the manufacturing process of the same.